Conventionally, to drive a rotary machine such an AC generator or a pump by a gas turbine, a uniaxial gas turbine having a simple structure has been mainly used. When the AC generator is to be driven by the unaxial gas turbine, as shown in FIG. 12A, the revolution speed of a gas turbine 101 reaching several tens thousands rotations per minute is reduced to a required revolution speed by a transmission 103, and an AC generator 102 is driven. In such a gas turbine generating system, it is known that, when the AC generator 102 is not in full power generation, that is, in a case of partial load, the fuel efficiency can be enhanced by reducing the revolution speed of the gas turbine 101.
FIG. 13 shows the revolution speed-fuel consumption characteristics of a gas turbine having a rated operation output of 275 PS. In this drawing, the specified revolution speed during the rated operation is assumed as 100% and the relation between the output for each of the numbers of rotations 105%, 100%, 95%, and 90% and the fuel flow rate is indicated by the curves L1, L2, L3, and L4. As clearly shown in the drawing, under the operation condition of smaller output than that during the rated operation, as the revolution speed is reduced from the specified revolution speed during the rated operation, the fuel efficiency is enhanced.
However, in a conventional gas turbine generating system, the transmission 103 is a gear transmission having a constant speed variation ratio, so that when the revolution speed of the gas turbine 101 is changed, the revolution speed of the AC generator 102 is also changed and AC power at a constant frequency cannot be generated. Therefore, in a conventional gas turbine generating system, even when the generator 102 is in a partial load, the gas turbine 101 is actually operated at a revolution speed of 100%. As a result, the following problems are imposed.
(1) To maintain the revolution speed of 100%, unnecessary fuel is consumed and the fuel efficiency is degraded.
(2) The inlet temperature of the turbine rises due to unnecessary fuel, so that the life of the turbine is shortened.
(3) Due to imperfect combustion of apart of unnecessarily consumed fuel, the atmospheric pollution is promoted.
Further, when a pump is to be driven by a uniaxial gas turbine, as shown in FIG. 12B, the revolution speed of a gas turbine 101 reaching several tens thousands rotations per minute is reduced to a required revolution speed by a transmission 103, and a pump 104 is driven. However, very large start torque is required to drive the pump 104 and the pump 104 cannot be started only by the uniaxial gas turbine. Therefore, in a conventional pump drive system, a huge fluid clutch 105 is required as a start clutch between the gas turbine 101 and the transmission 103.
However, in this system, when the pump 104 sucks mud, earth, and sand and the load is increased suddenly, it is effective to reduce the revolution speed of the pump and increase the torque. However, since the transmission 103 is a gear transmission having a constant speed variation ratio, a problem arises that the revolution speed cannot be reduced quickly in correspondence with a sudden increase of the load, and the gas turbine 101 is imposed to be stopped.